1. Field of the Invention
The present invention relates generally to an image formation apparatus for print forming full color images through superposing transfer of different color images by means of a plurality of electrostatic recording units having electrophotographic record printing facility of a printer, a copier, etc., and more particularly to an image formation apparatus capable of detecting and correcting positional offsets of color images between a plurality of removable electrostatic recording units.
2. Description of the Related Arts
Up until now, an electrophotographic recording type color printer is provided with four electrostatic recording units for black (K), cyan (C), magenta (M) and yellow (Y) which are arranged in tandem in the record paper conveyance direction. The electrostatic recording units for four colors optically scan a photosensitive drum on the basis of image data to form latent images, develop the latent images with color toners in developing vessels, transfer the developed images onto record paper being conveyed at a certain speed in a superposed manner in the order of yellow (Y), magenta (M), cyan (C) and black (K), and finally perform thermal fixing through a fixing device or other processes. In case of lack of color toners, the electrostatic recording units for yellow (Y), magenta (M), cyan (C) and black (K) must be subjected to replacement of the entire unit or of a part of the unit. To this end, the electrostatic recording units are each provided with a structure allowing easy removal with the apparatus cover opened.
On the other hand, to enhance the quality of the color printing in the color printer having such a structure in which the YMCK electrostatic recording units are arranged in tandem in the record paper conveyance direction, color matching accuracy must be enhanced by reducing the positional offsets of the toner images transferred onto the moving record paper by means of the electrostatic recording units. For example, if the resolving powers in the horizontal scanning direction (direction orthogonal to the conveyance direction) and in the vertical scanning direction (record paper conveyance direction) on the record paper are both 600 dpi, then the pixel pitch results in about 42 .mu.m, so that the positional offset must be reduced to 42 .mu.m or less.
In the case of the conventional tandem type color printer, however, it was difficult to achieve a color matching accuracy of 42 .mu.m or less in positional offset through improvement of the mechanical machining accuracy or assembling accuracy because the YMCK electrostatic recording units are removably provided and suffer from larger positional offsets as compared with the case of mounting in a fixed manner. To solve this problem in the case of Japanese Patent Laid-open Pub. No. Hei8-85236 for instance, test pattern resist marks are transferred on a transfer belt at four rectangular corners and read by a CCD to detect image offsets in resist mark detection coordinates relative to previously defined absolute reference coordinates of the apparatus so that upon the output of image data to the laser scanner, correction is made on the basis of image offsets whose output coordinates positions have been detected. In such conventional positional offset detection and positional offset correction, however, all of the electrostatic recording units for yellow (Y), magenta (M), cyan (C) and black (K) necessitate detection of positional offsets of the resist marks relative to the absolute coordinates. In addition, use of the CCD for the detection of the resist mark needs more time for the processing of positional offset detection and a larger hardware amount resulting in the increase of production costs. Although it is also conceivable to correct the positional offsets by allowing the light emitting array to be mechanically adjustable, such a mechanical adjustment suppressing the positional offsets to 42 .mu.m or less is unfeasible. For this reason, the color printer using the light emitting array often suffered from a positional offset as large as 300 .mu.m for instance, making it difficult to realize a satisfactory printing quality through the superposition of color components.
Thus, the present inventors proposed a method in which right and left resist mark trains are transferred on the belt in the order of black (K), cyan (C), magenta (M) and yellow (Y) so that the position of the marks is read for each color to calculate an image offset as a difference between the distance to the black (K) mark from the other color marks and its nominal distance (U.S. Ser. No. 09/086,956 filed May 29, 1998). However, this method involved a deficiency that a greater mark-to-mark distance may allow the offset attributable to the fluctuation in the belt conveyance speed to be contained in the detected distance, preventing accurate detection of the image offsets.